1. Technical Field
The present invention relates to a method and a device for producing a dispersion-hardened object which contains carbide nanoparticles. Furthermore, the present invention relates to a dispersion-hardened object which is produced with the method according to the invention such as, for example, a component for an internal combustion engine, preferably a piston ring.
2. Related Art
For piston rings, such as the ones of internal combustion engines with reciprocating pistons, a high wear resistance must be ensured because otherwise, i.e. in case of a low wear resistance, the coating becomes thinner. Thereby, the wall thickness of the piston ring decreases, the sealing effect gets worse, gas leakage and oil consumption increase, and the performance of the engine can get worse. Due to a piston ring that is subject to abrasion, the gap between cylinder wall and piston increases continuously so that it is easier for the combustion gases to escape by passing the piston ring (so-called blow-by) thereby reducing the efficiency of the engine. Furthermore, due to an increased gap, the remaining oil film which is not wiped off becomes thicker so that more oil per time unit can be lost and, thus, the oil consumption is increased.
In the field of thermal spraying of piston rings, today, preferably molybdenum-based materials are used by means of the plasma spraying method. However, the wear rate of the same in highly loaded engines is too high.
The high velocity oxygen fuel thermal spray technology (HVOF) offers the possibility to deposit particles with a low thermal influence and a high kinetic energy onto the substrate in such a manner that dense layers with high adherence are generated. To ensure, in addition, an improved wear resistance at higher loads, more recently, particles from metal carbides such as, for example, WC or Cr3C2 are used, which can not be sprayed by means of a plasma spraying method because they decompose at the high plasma temperatures of up to 20000° C. or form very brittle phases such as, for example, W2C. Said particles provide indeed a higher wear resistance; however, they have disadvantages due to their physical properties which are different with respect to the substrate, such as lower thermal expansion coefficient and lower thermal conductivity, and different mechanical properties such as lower ductility, i.e. higher brittleness and lower fracture toughness. These disadvantages have an impact during the engine operation, in particular in the range of mixed friction or insufficient lubrication. During these states, the thermal energy which is additionally induced during the friction results in a relaxing process in which the piston ring layer can not follow the expansion of the substrate due to the greatly differing thermal expansion coefficient and thus a network of cracks is generated. This effect results ultimately in a breakdown after repeated loads. Moreover, the metal carbides are usually introduced into a metallic matrix such as, for example, a NiCr alloy, wherein only a wetting of the alloy surface takes place but no metallurgical interlock is obtained. Thereby, the adhesion of the metal carbides such as WC or Cr3C2, which provide a high wear resistance as areas of hard material, is limited.
To increase the strength of a material, among other things, a dispersion hardening can be carried out. The particles present in this case form barriers for dislocation movements within the material during mechanical load. The dislocations generated and present during loading can not cut through the particles, in fact, they have to bulge between the particles. Dislocation rings are formed which, again, have to be bypassed. When bypassing, a higher energy input is necessary than during cutting. The yield stress for the traveling of the dislocation increases with decreasing particle distance and decreasing particle size. Therefore, the material strength increases as well.
A dispersion hardening would be possible by introducing carbides in the form of nanoparticles. The term “nanoparticles” relates here to particles with a size of 1 to 200 nm. The production of nanocrystalline thermal spray coatings has previously been carried only by means of agglomerated nanoparticles. Such agglomerates of nanoparticles can reach a diameter of 0.1 to 100 μm. Only with particle sizes larger than 1-2 μm, the particle transport under normal pressure conditions is possible. Due to the fact that, for a directed transport in a gas flow, nanoparticles have to absorb a minimum amount of energy through the collision with the gas molecules and that the energy maximally to be absorbed decreases with decreasing particle size, the nanoparticles can be transported in a directed manner only up to a minimum size. This would only be possible through lower process pressures or through electrically charging the particles. In particular at particle sizes below 800 nm, particles behave like gas molecules. A nanocrystalline HVOF layer thus can only be produced if agglomerated nanocrystalline powders are available. A particle reinforcement thus has to be carried out already within the powder. This results in that the generated coating contains microparticles and agglomerates from nanoparticles, but no finely dispersed discrete nanoparticles. Coatings containing agglomerates of nanoparticles are described, for example, in DE 10 2007 018 859 A1, DE 100 57 953 A1, U.S. Pat. Nos. 5,939,146 A, 6,723,387 B1 and US 2004/0131865 A1.